Elastomeric polypropylene has been prepared by G. Natta et al. (U.S. Pat. No. 3,175,999) using TiCl.sub.4 /AlEt.sub.3 catalyst, with the resulting material containing 90% isotactic polypropylene. Collette et al. claimed in U.S. Pat. No. 4,335,225 the usage of homogeneous zirconium or hafnuim catalysts supported upon partially hydrated alumina to produce a polymer comprising 55% or less isotactic polypropylene with properties similar to the earlier product of Natta et al. Much higher activity catalysts were patented by Wilson et al. (U.S. Pat. No. 4,971,936) and by Job (U.S. Pat. No. 5,270,410) to prepare polypropylenes containing primarily isotactic (50-70%) and predominently syndiotactic structures, respectively. The lengths of the stereoregular sequences are only about 10 monomeric units. All the heterogeneous catalysts employed in the above inventions, however, are ill-defined, the polymers obtained are difficult to be characterized, and the elastomeric properties have proven inadequate for useful application, particularly with regard to tensile set.
It has been disclosed by applicant in Chien et al. U.S. Pat. No. 5,756,614 that, using an asymmetric stereorigid metallocene possessing two catalytic sites with different stereochemical control modes, a uniform well-defined stereoblock polypropylene can be prepared having long isotactic and atactic sequences alternating in the same macromolecule. Such material is an excellent thermoplastic elastomer with low tensile set and hysteresis. A somewhat similar material was also obtained using slowly rotating nonrigid metallocene (U.S. Pat. No. 5,594,080). These catalysts however, while highly useful are expensive to manufacture, relatively low in productivity, and providing materials with a somewhat limited range of properties.
The applications of a thermoplastic elastomer depend on the melting and glass transition temperatures of the materials, (T.sub.m and T.sub.g), the tensile modulus, optical clarity, elongation to break, and the permanent set properties. They are determined by the percentage content of crystalline and amorphous polymers, the steric purity of the former, their molecular weights and the effectiveness of coupling between them. Different molecular structures of catalyst indeed, are required to synthesize a polymer with particular sets of properties. One way to accomplish this end is to select two metallocenes differing in stereoselectivity and rates of monomer insertion. This is not done, however, because normally, two different polymers are immiscible. This is true for different polyolefins assembled from the same monomer molecule having different geometrical, chemical, or stereochemical isomeric structures. A well-known example is low density polyethylene manufactured at high pressure and high density polyethylene manufactured at low pressure. Other prior art examples include the products of first generation Ziegler-Natta catalyzed propylene polymerization which include high molecular weight crystalline isotactic polypropylene and lower molecular weight amorphous atactic polypropylene. The two polymers are immiscible and the amorphous polymer must be removed since its presence renders the crystalline polymer physically and mechanically too weak to be of any commercial value.
Sometimes, however, two different polymers can be forced to form a compatible blend by thermomechanical means. This is generally not usually economically acceptable in view of added processing cost and degradation of the polymers. More usefully, two different homopolymers can form a compatible blend with the aid of an additive which can be a block or graft copolymer of the two homopolymers. One of the problems associated with the prior art agents and methods of blending is that it is not a simple task to devise a commercially viable synthetic method for its preparation and subsequently to blend the components into a homogeneous material without phase separation. This objective is difficult to achieve because of the short chain life times in Ziegler-Natta catalysis. This invention relates to a process providing the "one-pot" direct synthesis of "naturally" compatibilized thermoplastic elastomeric polyolefin alloy.
Well-defined organometallic compounds, such as Group IVB elements of the Periodic Table (Handbook of Chemistry and Physics, 49th Edition, Ed. R. C. Weast, Chemical Rubber Co. Cleveland, 1968) have been found to possess stereoselectivity in the polymerization of propylene depending upon the ligand structure of the catalyst precursor.
For example, in one prior art method, chiral group IVB metallocene precursors act as catalysts for the isospecific polymerization of propylene to yield isotactic polypropylene, (See U.S. Pat. No. 4,794,096 and the articles by Kaminsky et al. Angew. Chem. Int. Ed. Engl. 1985, 24, 307 and by Ewen in J. Am. Chem. Soc. 1984, 106, 6355).
In addition, Ewen et al., as disclosed in J. Am. Chem. Soc. 1988, 110, 6255 and U.S. Pat. No. 4,892,851, taught that zirconocene precursors having bilateral symmetry could produce syndiotactic polypropylene and are capable of polymerizing ethylene, .alpha.-olefins and cycloolefin with high activity.
Organometallic compounds having C.sub.2v symmetry, whether in the form of a stereorigid zirconocene or a free rotating complex (Chien et al., Macromolecules 1995, 28, 5399), tend to catalyze propylene polymerization without profacial selectivity. Similar nonspecific polymerizations of propylene have previously been catalyzed by titanium complexes with either a single .eta..sup.5 ligand or a phenolic ligand.
All of the above precursors are activated by a cocatalyst which transforms the former catalyst into the corresponding cationic species (See U.S. Pat. No 5,198,401 and EP 573,403). The cocatalyst comprises a cation which irreversibly reacts with at least one ligand from the Group IVB metal complexes to form the catalytically active cationic Group IVB complex. The counter anion is non-coordinating, readily displaced by a monomer or solvent, has a negative charge delocalized over the framework on the anion or within the core thereof, is not a reducing or oxidizing agent, forms stable salts with reducible Lewis acids and protonated Lewis base, and is a poor mucleophile.
Other prior types of cocatalyst include Lewis acids which will irreversibly react with at least one ligand from a Group IVB or VIII metal complex to form an anion possessing many but not all of the characteristics detailed above (See Marks et al. J. Am. Chem. Soc. 1991, 113, 3623).
The cocatalyst which is more commonly employed than the two types mentioned above, is methylalumoxane. Methylalumoxane acts not only as a Lewis acid, but also serves in other useful functions as well.
High molecular weight polypropylene, having a certain steric structure, prepared individually in the presence of one of the prior art catalysts described above, is generally immiscible with another polypropylene of a different structure. For example, a solvent-cast blend of any pair of stereoisomeric polypropylenes, e.g., isotactic and atactic, or syndiotactic and atactic, tends to crumble easily, and the tensile bar press molded from the blend fails with the least bit of strain.
In another prior process, solutions of two different metallocenes are used to polymerize ethylene as if each is unaffected by the presence of the other. This method is useful for preparing polyethylenes with bimodal molecular weight distribution using two Group IVB metallocenes as disclosed by Ewen (Studies in Surface Science and Catalysis Vol. 25 Catalytic Polymerization of Olefins Eds. Keii et al., Kodansha, Elsevier, 1986, pp.271), and by Ahlers and Kaminsky (Makromol. Chem.; Rapid Commun 1988, 9, 457). The gel permeation chromatograms of the produced bimodal polyethylene are exact superposition of chromatograms for a mixture of polyethylene obtained with the two different metallocenes separately. A polypropylene having multimodal molecular weight distribution was obtained using an ansa-hafnocene and ansa-zirconocene mixture to produce isotactic polypropylenes, albeit having molar masses that are different.
Despite all of the prior processes for preparing various polymers, up until the discovery underlying the present invention, there has been no process that is capable of forming compatibilized crystalline and amorphous polyolefin alloys of the present invention. Unlike the prior art, the present invention allows one to synthesize, directly in a "one-pot" polymerization of a single monomer, useful alloys of semicrystalline and amorphous polyolefins having different steric structures without the need for subsequent blending of the polyolefins--providing such novel "one-pot" direct synthesis of "naturally" compatibilized thermoplastic elastomeric polyolefin alloy.
More specifically, the present invention provides novel thermoplastic elastomeric polypropylene alloys comprising three well-defined components: a homopolymer of stereoregular polypropylene, a homopolymer of atactic polypropylene, and a copolymer having atternating sequences of both types of stereoisomeric structures, and catalyst components useful in the production of these alloys, and with a process using such catalysts to provide independent control over the formation of the three well-defined components.